1. Field of the Invention
This invention relates in general to digitizing tablet construction and specifically to providing shielding to prevent electrostatic noise interference from adversely affecting the output signal from a digitizing tablet.
2. Description of the Related Art
Digitizing tablet sensor circuits can be adversely affected by ambient electrical and electrostatic noise from light sources, display monitors, and nearby electrically operated devices. Severe interference can be caused by image generators such as television or computer monitor screens when a digitizing grid is placed over the screens for digitizing a generated image.
To reduce this interference, prior art systems insert a conductive shield between possible noise sources and the sensors. The shield is grounded to conduct electrical noise away. Several examples of the prior art illustrate this approach to eradicating unwanted noise and electrostatic interference.
U.S. Pat. No. 4,644,102 shows the use of an electrostatic shield using grounded copper strips vice a grounded copper plate. The grounded copper strips prevent the effects of reversed fields that occur when using a conventional grounded plate. Noise is eliminated or at least reduced by the use of a demodulator coupled to a matched filter instead of a conventional low-pass filter which was considered to have too slow a recovery time.
U.S. Pat. No. 4,636,582 teaches the use of a conductive shield layer coupled to ground to prevent electrostatic interference with handwritten information detected by a pair of conductive layers activated by a writing stylus. Electrostatic noise from the hand of a user or nearby electrical devices interferes with the decoding of signals representing entered information. The shield is intended to ground at least some of the electrical noise.
U.S. Pat. No. 4,497,977 shows the use of an electrostatic shield layer coupled to ground to prevent noise from the hand of a user from causing interference with electrical signals representing information entered via a handheld stylus.
U.S. Pat. No. 3,974,332 describes a system for entering information using a conductive pen capacitively coupled to a tablet for selecting segments on the tablet. The segments are insulated from one another by an electrically conductive grounded shield layer with apertures for the segments. The shield prevents capacitive coupling to segments adjacent to a selected segment, i.e., prevents or at least reduces crosstalk between segments.
U.S. Pat. No. 3,732,369 shows a digitizing tablet having x- and y-coordinate wires that are capacitively coupled to a stylus for supplying signals indicative of the x,y position of the stylus. A conductive ground plane, insulated from the lower wires, is provided.
U.S. Pat. No. 4,956,526 describes the use of a shielding plate placed at a distance from a sensor plate to provide a maximum output voltage signal.
U.S. Pat. No. 4,853,497 teaches the use of pan-shaped nonferrous shield that extends over and close to the sides of a rectangular grid of wires in an electromagnetic digitizer to eliminate edge effects.
U.S. Pat. No. 5,124,509 describes a digitizing device having a conductive plate in an inductively coupled system. The purpose of the plate is not to eliminate or to reduce interference noise. It operates to detect a submultiple of the stylus' oscillator frequency, the latter being used to induce a voltage signal in the associated grid lines. The submultiple frequency is capacitively coupled from the stylus to the user's hand or body. The plate picks up the submultiple frequency and couples it through a phase-locked loop to operate an analog switch. The conductivity of the plate is purposely made low. Also shown is a prior art standard grounded electrostatic shield.
U.S. Pat. No. 5,218,173 relates to materials used in shields for electromagnetic digitizers such as used in notebook computers. The shield plate protects the circuitry from local interference and magnetic fields such as those created by power transformers and the like.
Although accomplishing their purpose, none of the prior art shields are used to supply a noise cancellation signal as shown in the present invention described below in more detail.
These prior art shields are not completely effective, especially in those cases where the shield must be transparent (for placement on an image plane or monitor display screen, for example) while exhibiting low resistance to electrical potentials. The invention to be described improves the performance of electrostatic shielding by cancelling noise from the output signal.